The unit bug that crashed into Mars

On September 23, 1999, NASA lost a $327 million spacecraft to a unit conversion. The Mars Climate Orbiter’s ground software, written by Lockheed Martin, reported thruster impulse in pound-force seconds. The trajectory software at JPL read those numbers as newton seconds. Every small correction burn was silently wrong by a factor of 4.45, for months, until the orbiter hit the Martian atmosphere 170 km too low and broke apart.

Nothing threw and nothing asserted. The numbers were just numbers.

Making the bug a type error

That is the class of bug mp-units exists to kill. It is a modern C++ library (C++20 and later) in which a value’s unit is part of its type, so a pound-force second and a newton second cannot silently mix: they either convert exactly and explicitly, or the code does not compile.

Two things to notice in that demo. The conversion small_forces.in(N * s) produces 444.822 N s from 100 s lbf, the exact factor-4.45 relationship that killed the mission, now handled by the type system instead of by a human remembering to multiply. The commented-out last line assigns a bare 100.0 to a newton-second quantity, and it is a compile error. There is no path from “raw number” to “quantity” without saying what the number means.

None of this costs anything at runtime. The dimensional analysis happens entirely at compile time; what remains in the binary is the same arithmetic you would have written by hand, minus the opportunity to get it wrong.

What the rest of the series covers

mp-units goes deeper than attaching a unit to a double. It models the International System of Quantities (ISO 80000) faithfully enough to catch bugs that dimensional analysis alone cannot see, and I will show those in the next post.

It is also on the standard track. The library is the reference implementation behind P3045, “Quantities and units library,” a candidate for C++29, so reading mp-units today previews what may land in std itself. Its lead author is Mateusz Pusz, a C++ architect from Gdańsk and an ISO C++ committee voting member since 2017.

Over the coming weeks I will walk through the library’s big ideas, one runnable demo at a time: quantity kinds, the affine space, one-line custom units, unit-safe formatting, and the road to C++29.

Sources: mp-units on GitHub · documentation · NASA, Mars Climate Orbiter Mishap Investigation Board Phase I Report (1999).